The OT? The power tube(s)? The preamp? All of the above? I just started power testing this week. Tried it on a couple different amps. What I found. There is no real technique to finding the max power signal. Any reasonable signal level, frequency, or amp settings will do. The wave is nicely shaped until it reaches its limit or ceiling and then it just conforms to that level and does the hard level cut. Where does the hard clip usually occur in these kind of tests? First I was measuring a champ. Got it up to 5 Watts on the scope and figure it must be the power tube getting close to its 6W limit. Then I was looking at a 12 watt Princeton. It gets up to 10-11 W on the scope. Is that the tubes reaching their limit (12 W each) or is it the undersized output transformer reaching its limit? The power tubes are biased cool, but they could be biased just as hot as a hot biased 22W deluxe reverb. Back to the champ. I was able to get distinctly different breakup on either end of the wave. The hard straight clip must be saturation and the angled clip must be cut off. Saturation happened right before cutoff, meaning the bias could be cooled a little. How is it certain the hard clip is saturation and not cutoff? Most likely because the Princeton has the hard clip on both ends of the wave and that is saturation. Not sure why the cutoff is not visible at all on the Princeton. So I’m pretty sure the hard clip is power tube saturation. To make sure I scope the power tube itself. I have measured signal on the grid and plate before with a DMM so I had some idea what to expect. I was going to scope the plate of the power tube, expecting a couple hundred volts but then I noticed the voltage scale setting on the O-scope only goes up to 20 V/cm so I ruled that out. So I scoped the grid. I had the idea that when you scope different places on the amp it always looks the same. This is probably not true because signal tracing would not work. What did the grid look like on the scope? It looked weird. There were three signals instead of one and they were at an angle, but it still did the hard clip at high volume. This is the real question that necessitated this post. If I saw a hard clip on the grid of the power tube, does that mean that the clip started in the pre amp?